Suggestion: hook into multipart form field parsing

Assume a multipart form with the following fields: - csrf - projectId - files With the current `@hattip/multipart` parsing you have no way to check the csrf, before processing the files. You also have no way to access the projectId in the `handleFile` function, which is tedious when you wanna include that projectId in the resulting file path. Currently the only way is to either provide csrf and projectId via headers or query parameters, or to store the files in a temporary directory. Idea: add options to control the order of the parsing and to hook into fields. Also add the already processed fields as a second parameter of `handleFile`. ```tsx let projectId = ''; const formData = await parseMultipartFormData(request, { // not all fields have to be in the order, missing fields will be processed at the end order: ["csrf", "projectId"], async handleField(name, value) { if (name === 'csrf' && value !== PROPER_CSRF) { throw new Error("Access denied"); } if (name === 'projectId') { projectId = value; } return value; }, async handleFile(info, fields) { // access projectId via "fields", or via the "projectId" variable from above ...upload things } }) ```